Four 'luffing jib' cranes at work on the new Amex building situated just behind the 'old' one. The choice of this type of crane must be due to the size of the site. A single horizontal jib could not cover the area and four would interfere with each other. By choosing luffing jibs four cranes can work simultaneously and thus shorten construction times.
(I am not a civil engineer. I wait to be 'shot down' by those who know about these things.)